Bookmark and Share

Basic crystallography in the PC era

The recent XIX IUCr Congress in Geneva featured the 'rediscovery' of twins by molecular crystallographers. The number of communications concerning polytypes in organic and inorganic compounds is constantly increasing, although some authors improperly call them 'polymorphs'. And allotwins (twins of polytypes recently defined) are attracting attention (e.g. the poster No. 46 in the second poster session). The data on these topics is seldom accompanied by crystallographic knowledge. This ignorance comes from the fact that the basic texts on the topic are not in English (ex. Friedel’s classic French texts on twins, and Dornberger-Schiff’s lectures in German on OD and polytypism). Twins and polytypes were gathered in a microsymposium under the ironic category of 'demons'. Nowadays courses of basic crystallography are rarely offered to future crystallographers except at universities where a mineralogical influence is still present. Consequently the solution of 'demon' structures is entrusted to 'blackbox' software packages requiring just a bit of experience in using a computer. The user is untutored in the process of recognition and treatment of twinning. This is not surprising when viewed in the context of the transformation of the crystallographerscientist into a technician-button-pusher. But it has additional costs. Use of a polarizing microscope and critical analysis of space-group absences to detect twinning should precede data collection and structure determination rather than be a post-solution procedure. The user uncritically accepts the robotic post-solution analysis. Analysis of polytypes is even worse, because streaks accompanying families of reflections are usually ignored resulting in loss of vital information, because the OD theory could easily offer an explanation and even reveal undiscovered polytypes (incorrectly termed 'polymorphs'). Some contributions at crystallographic meetings are characterized by arbitrary, ambiguous, confusing, and contradictory nomenclature. These reports often fail to document the analysis strategy that was followed. Despite this dark landscape, the symposia, workshop and satellites about 'software usage' flourish. While nobody can deny the assistance these packages offer the crystallographer, the time has come to question whether one also needs schools for basic crystallography, to insure informed use of the programs. Some comprehensive textbooks dealing with basic crystallography have been published recently, and should be popularized more extensively. Finally, the education of a knowledgeable crystallographer should be considered a task of primary importance for the IUCr. Before it is too late . . .

Massimo Nespolo and Giovanni Ferraris